Add cellular on iPad with my existing cell phone

No they aren’t equipped to be used as cell phones. Tablet sims are for data only 

Setup your AppleID account to use FaceTime audio on your iPad via WiFi. I use my iPad via WiFi-C to make/receive calls when I'm too lazy to grab my phone or my laptop. It's not the same as using your cellular account but it works just fine.

If you mean put your phone SIM card into the iPad, no. You can of course add a line to your account for the iPad.